Digital selective transformation and patterning of highly conductive hydrogel bioelectronics by laser-induced phase separation

The patterning of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hydrogels with excellent electrical property and spatial resolution is a challenge for bioelectronic applications. However, most PEDOT:PSS hydrogels are fabricated by conventional manufacturing processes such as photolithography, inkjet printing, and screen printing with complex fabrication steps or low spatial resolution. Moreover, the additives used for fabricating PEDOT:PSS hydrogels are mostly cytotoxic, thus requiring days of detoxification. Here, we developed a previously unexplored ultrafast and biocompatible digital patterning process for PEDOT:PSS hydrogel via phase separation induced by a laser. We enhanced the electrical properties and aqueous stability of PEDOT:PSS by selective laser scanning, which allowed the transformation of PEDOT:PSS into water-stable hydrogels. PEDOT:PSS hydrogels showed high electrical conductivity of 670 S/cm with 6-μm resolution in water. Furthermore, electrochemical properties were maintained even after 6 months in a physiological environment. We further demonstrated stable neural signal recording and stimulation with hydrogel electrodes fabricated by laser.

The relative intensity of the Gaussian beam at the depth of zR and position of r = 3 μm was calculated as 0.24 which is higher than the threshold point of LIPSP.
The Gaussian beam profile was also plotted at the focal plane (i.e., z = 0 and w(z) = w0) and at the Rayleigh length (i.e., z = zR and w(z) = wR) ( fig. S9D). The intensity of the Gaussian laser beam at zR and the position of r = 3 μm was revealed to be higher than that of the intensity of the beam at the focal plane ( fig. S9D, (i)) which enables phase separation of PEDOT:PSS at the corresponding depth level.
The depth of the surface where we processed the PEDOT:PSS with a thickness of 10 μm was also studied. With a thickness of 10 μm, PEDOT:PSS hydrogels can secure sufficiently high electrochemical and electrical properties. In this case, the depth from the center of the PEDOT:PSS sample to the surface becomes 5 μm. It showed that there was little difference in the intensity between the center and the surface ( fig. S9D, (ii)). Finally, we demonstrated that PEDOT:PSS with a thickness of about 10 μm was uniformly processed by the LIPSP (fig. S9E, F, and G).

Calculation of water contents of swollen PEDOT:PSS hydrogels
To calculate the swelling ratio and water contents, the thickness of micropatterned PEDOT:PSS hydrogels in the fully dried state and swollen state were measured through a 3D surface profiler (NANO View-E1000, Korea). The samples were fully dried at room temperature for 1 day before measurement. After measuring the thickness of dried samples, they were immersed in DI water for 1h, then the swollen thicknesses were taken. As a result of the experiment, definite anisotropic swelling behaviors were observed ( fig. S14A), thus lateral expansion rarely occurred. Therefore, only the change of thickness in z-direction was considered in the volume change of PEDOT:PSS hydrogels. We modeled the PEDOT:PSS hydrogels as a cuboid and hypothesized that the volume change of the PEDOT:PSS hydrogel was solely due to water absorption ( fig. S15). The volume of PEDOT:PSS in the dried state and the swollen state can be obtained simply by multiplying the lengths of the three sides. Where x1, y1, and z1 are lengths in the dried state and x2, y2, and z2 are in the swollen state.
Since the lengths in the x-and y-axis does not change due to anisotropic swelling of PEDOT:PSS hydrogels, the swelling ratio and the change of volume due to water absorption are only dependent on thickness change in the z-direction. Therefore, the swelling ratio and water contents of PEDOT:PSS hydrogels can be obtained by simple calculation of measured thickness of PEDOT:PSS hydrogels as follows.                Since the CIC values are dependent on the geometric surface area (GSA) of electrode material, we tested two different electrodes of GSA for comparison. PA 10 electrode with GSA of 165,000 μm 2 showed the current density of 473 mA cm -2 and 1,019 mA cm -2 for GSA of 70,300 μm 2 which is much higher than conventional metallic electrodes such as platinum and gold. CIC was then calculated by integrating chronoamperometric curves, which showed 7.45 mC cm -2 for GSA of 165,000 μm 2 and 16.17 mC cm -2 for GSA of 70,300 μm 2 .    Table S1. Comparison of the LIPSP with preceding studies in terms of the conductivity and spatial resolution in aqueous environments.